Social media seems to be the answer these days. Do you have a twitter account, facebook page, or other social media platform dedicated to your classified site? That could be a great way to advertise. Just be sure to post genuine content along with the salsey content.
What about guest posting on a popular blog in your niche? Making a guest post should lead to backlinks and it will make you stand out as an instant "expert" in your field. Blogs that focus on family, finances, employment, or sustainability/recycling, could all be potential targets. When you make your pitch, be sure to tell the blogger how your site will help their audience members. Let them know what makes your site better than the others.
If buying ads is beyond your budget, another option would be trade an advertising spot with another non-competing business. You run an ad for them, they run one for you. Good luck!